1. Field of the Invention
This invention relates to a novel machine tool cutter and particularly to a broaching tool in which the cutting surfaces are at a positive rake angle to the work cut.
2. Description of the Prior Art
The typical broaching tool is in the form of an elongated body having a plurality of spaced annular ribs generally transverse to the longitudinal axis of the body, these ribs forming a series of annular cutters. Each annular cutter has a cutting edge and adjacent chip relief groove and normally engages the work cut on the work piece at a positive rake angle.
The usual designs have placed tremendous stresses on the tool and the cutters as well as on the work piece being broached. There have been various attempts in the past to develop new designs which will decrease the load on individual cutters, such as the tooth design shown in U.S. Pat. No. 3,849,852, issued Nov. 26, 1974. That design in theory decreases the load because the individual tooth takes a narrow deep cut rather than a wide shallow cut.
U.S. Pat. No. 1,945,535, issued Feb. 6, 1934, describes a broaching tool having cutters with a positive rake angle and a specially designed chip relief groove. The cutter at a positive rake angle will in theory take a cleaner cut and leave a smoother surface behind the cut by rolling the chip formed into the chip relief groove, thereby reducing fracturing off of small particles and serving to reduce generation of heat in the work and in the tool.
As a tool cutter is forced through a work piece, high friction is generated between the tool face and the compressed material adjacent the tool face. With the high temperature and high specific pressure present, a layer of material clings to the tool face to form what is known as a built-up edge. This built-up edge is, therefore, forced through the work by the cutter face and does the actual fracturing or cutting. The chip body shears away from this built-up edge and passes off above.
As the built-up edge becomes larger and larger, it becomes more and more unstable until fragments thereof are torn off. The built-up edge is, therefore, continually varying in size and attacking the area of fracture or shear with a varying area which causes roughness. Furthermore, as the built-up edge increases in size, the chip being formed and following upwardly of the tool face is subjected to a band of increasing curvature which would remain at a substantially constant value if the built-up edge can be kept at a reduced size.
Such problems were recognized by the inventors in U.S. Pat. No. 2,392,481, issued Jan. 18, 1946, but they continued to work on the size and shape of the chip relief grooves much in the manner as was done by the inventor in U.S. Pat. No. 1,945,535.
None of these prior designs was able to solve the problem of the immense pressures and built-up edge which caused the cutting edge to quickly wear and frequently break with the resultant rough work piece surface behind the cut.
In Proulx et al U.S. Pat. No. 3,946,472, issued Mar. 30, 1976, there is described a broaching tool in which the cutter teeth are in the shape of buttons or discs. This had the unique feature of being able to make use of the angular position of the discs so as to form a smoothly curving contoured surface. This represented a very significant advance in the art in being able to greatly reduce the power required per unit volume of chip removed. The result was less wear on the individual cutters, less breakage of cutters and less damage to the work piece being machined.
It is, therefore the object of the present invention to provide an improved configuration of tool cutter which can take advantage of the benefits of U.S. Pat. No. 3,946,472 while providing a positive rake angle between the cutter face and the work cut.